In the acid-catalysed esterification of a carboxylic acid with an alcohol, the first step is the protonation of the carbonyl oxygen of the acid. What is the role of this step?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → It activates the carbonyl group towards nucleophilic addition of the alcohol.
The carboxylic carbon is only weakly electrophilic, and an alcohol is a weak nucleophile. When the acid catalyst puts a proton on the carbonyl oxygen, the oxygen carries a positive charge and pulls electrons from the carbon, so the carbon becomes more electrophilic and the alcohol can add to it, giving the tetrahedral intermediate:
\( \displaystyle \ce{RCOOH + H+ <=> RC(OH)2^+} \)
Why the other options are incorrect
Option (1) It makes the carbonyl carbon less electrophilic. — Incorrect. It does the opposite: the positive charge on oxygen makes the carbon more open to attack.
Option (3) It removes the α-hydrogen of the carboxylic acid. — Incorrect. A proton is added to the carbonyl oxygen; no α-hydrogen is involved, and the alkyl part of the acid is not changed.
Option (4) It converts the alcohol into an alkoxide ion, which is a stronger nucleophile. — Incorrect. An alkoxide ion forms in the presence of a base, not an acid. The neutral alcohol molecule is the nucleophile here.
NCERT (2026-27 edition):
8.9.2 Reactions Involving Cleavage of C–OH Bond – “Mechanism of esterification of carboxylic acids: The esterification of carboxylic acids with alcohols is a kind of nucleophilic acyl substitution. Protonation of the carbonyl oxygen activates the carbonyl group towards nucleophilic addition of the alcohol.”